Nonlinear spontaneous symmetry breaking in active polar films

The behaviour of an active polar suspension in a fluid film is analysed in the vanishing Reynolds number limit. We perform a detailed study of the transitions to spontaneously flowing steady states and their associated density variations. Beyond the onset of instability, we find a new transverse symmetry breaking of the flow generated in the film. This spontaneous symmetry breaking is observed despite symmetric boundary conditions (w.r.t orientation) on either side of the film. We study this new phenomenon by means of numerical simulations and nonlinear theory, showing that it can be ascribed to the nonlinear coupling, characteristic of polar active systems, of the density gradients with the flow velocity and the orientation field. As such a theoretical analysis based on linear stability arguments typically used to identify phase boundaries is unable to describe it. An extension of the theory to allow for higher density regimes is also proposed.